Magnetic Resonance Imaging (“MRI”) devices operate by establishing a fixed, steady magnetic field, together with a variable gradient field, applying a radio-frequency (“rf”) signal to the volume being investigated, and detecting an rf response. They therefore need to operate within a volume that is free from external rf signals. In principle, this is easily achieved by placing the device within a Faraday cage.
As the rooms provided for MRI devices were often not originally designed with this specific purpose in mind, this is usually achieved in practice by creating a custom wooden enclosure immediately within the walls of the room, which is then covered with copper sheet. This is convenient in that the wood can be trimmed to the available space using well-known carpentry techniques, and the flexible nature of the copper sheeting allows it to be tailored to the wooden construction.
This approach achieves the necessary technical standards, but is problematic in that wooden structures can be difficult to bring within fire regulations. Further, as more complex devices are introduced, such as those combining MRI and radiotherapeutic functions, the necessary shape of the Faraday cage becomes more complex and the use of a bespoke construction of this type becomes less practical.